1. Field of The Invention
The invention relates to a magnetic disc memory, comprising a device for positioning a magnetic head on a track, a disc being sub-divided into sectors comprising groups of data tracks and sectors comprising groups of servo tracks in an alternating manner, said servo tracks defining the data tracks which are staggered over one half track pitch with respect thereto, each servo track containing magnetization changes (so-called reference transitions) for reference purposes, the servo sectors comprising, per group of servo tracks, step-wise magnetization changes (so-termed measuring transitions) from track to track for position detection purposes, said magnetization changes generating signals in the magnetic head which, when applied to a detection circuit and an amplitude control circuit, supply information with regard to a track number reached in a group and which provide positioning of the magnetic head between two adjacent servo tracks.
2. Description of The Prior Art
A device of this kind, including an associated magnetic disc, is known from U.S. Pat. No. 3,812,533 which extensively describes an embodiment of a disc memory in which the servo signals are recorded in some 130 sectors on all disc sides. The tracks are sub-divided into groups of eight tracks. The positioning onto a track is effected by means of two separate systems: coarse positioning is effected in this embodiment by means of a coarse-control servo loop which includes an optical position transducer, while fine positioning is realized by means of said servo signals. This invention concerns fine positioning, which is why special attention is paid to fine positioning hereinafter.
After a carriage supporting the magnetic head or heads has been positioned on a given group by coarse control, positioning is taken over by fine control; the following then occurs: the reference transitions of one or two neighboring servo tracks in the group supply a reference pulse after detection. The measuring transition (transitions) of this servo track or those of nighboring servo tracks generates (generate) a measuring pulse (measuring pulses) in the read-write head. Measurement of the time expiring between the reference pulse and the meaasuring pulse (pulses) indicates on which track or in the vicinity of which track the head is situated. Determination of the difference between the amplitudes of the two measuring pulses (if one pulse is lacking, its amplitude is assumed to be zero) accurately indicates the relative position with respect to the two associated servo tracks. Both data are used for position control. The measuring transitions, and hence the measuring pulses derived therefrom, all have the same sign in the known device and form a regular pattern. As a result, a low-frequency crosstalk signal arises in the magnetic head, said crosstalk signal originating from tracks which are situated further away. The measuring pulses are superposed thereon, which is liable to cause incorrect positioning. This constitutes a serious drawback of the known device.
A further drawback of the known device is that the measuring transitions of like sign must be situated at a suitable distance from each other in successive servo tracks in order to enable discrimination in a comparison circuit which forms part of the control circuit. A further minimum value for the measuring transition interval in successive tracks is imposed by the time required for performing some electronic switching functions, because the amplitudes of two successive measuring pulses, originating from the signal of the magnetic head situated somewhere between two servo tracks and having the same sign, must be determined by different peak detectors. These distance requirements are the reason that the length of the servo sectors in the tracks is subject to a strict minimum. A practical figure in this respect is a time interval of 1 .mu.s between successive measuring transitions, which means a distance of 50 .mu.m for a rotary speed of 50 m/s. If extension of the groups of tracks were contemplated, this would imply a substantial loss of data storage capacity. However, extension of the number of tracks per group is attractive (for example, from 8 tracks to 16 or 32 tracks per group), because the requirements to be imposed as regards coarse control may be substantially less severe, or because even simpler and cheaper methods of coarse control (for example, a mechanical or magnetic system instead of an optical system) may then be adopted.